The invention relates to a method for correcting errors in a digital imaging system having an image sensor and a readout apparatus for reading out the image sensor, in which a dark current image is captured using the image sensor and the readout apparatus. Moreover, the invention relates to a digital imaging system with an image sensor and with a readout apparatus for reading out the image sensor, wherein the readout apparatus is configured to capture a dark current image with the image sensor.
Digital imaging systems are well known and are used for various applications. For example, a digital camera or a digital video camera comprises such a system.
The image sensors used in the digital imaging systems, such as a charged-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) sensors, frequently have inhomogenous dark current noise in the sensor plane. This relates to both linear image sensors such as those used in line scan cameras, as well as flat image sensors such as those used in cameras. The image captured by a completely dark image sensor frequently manifests a weak pattern of stripes. The inhomogenous capturing characteristic of the image sensors arises from production-related tolerances and systemic characteristics of the employed camera electronics.
For end-user applications, the artifacts that arise in the captured images, which are manifested in particular as a slightly varying image brightness, tend to play a subordinate role. For professional applications such as imaging in microscopy, such systematic image errors are however, undesirable. The differences of the background signal in the sensor plane reduce the signal-to-noise ratio and overlap the weak light intensities of the captured images.
In order to correct the systematic image errors, a corrective image is taken for the individual sensor in a calibration process. During the calibration process, the image sensor is completely darkened, e.g., covered. As a corrective image, a so-called dark current image is then taken that reproduces the dark current noise of the image sensor. Then, this corrective image specific to the sensor is removed from the images captured with this sensor. This form of calibration is possible since, when the sensor is completely darkened, the arising image errors do not depend on the amount of incident light or the quantum efficiency of the individual pixels of the image sensor. The sensor errors add up as they overlap the captured image.
The corrective image is normally stored in a memory of an image processing unit coupled to the camera, such as a hard disk of a computer. This corrective image is then removed from each image captured with the assistance of the imaging system, for example by the image processing unit executing a program.
The object of the invention is to present an improved method for correcting the errors in a digital imaging system and to present an improved digital imaging system, wherein the design complexity of the digital imaging system is minimal.